The original impetus for the open licensing of the d20 System was the economics of producing role-playing games. Game supplements suffered far more diminished sales over time than the core books required to play the game. Ryan Dancey, Dungeons and Dragons' brand manager at the time, directed the effort of licensing the new edition of Dungeons and Dragons through the 'd20 System Trademark', allowing other companies to support the d20 System under a common brand identity. This is distinct from the Open Game License, which simply allows any party to produce works composed or derivative of designated Open Game Content.

Theoretically this would spread the cost of supplementing the game and would increase sales of the core books, which could only be published by Wizards of the Coast under the Dungeons and Dragons and d20 System trademarks. (To this end, the SRD does not include rules for character creation and advancement.) The marketing theory behind the d20 System and its associated licenses is network externalities; support for the core rules would become an external expense rather than one incurred by Wizards of the Coast, but would promote the sales of the company's core rulebooks.

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Mechanically speaking, the d20 System is similar to numerous older, proprietary game systems. The most obvious example is the D20-based Megaversal system used by publisher Palladium Books; d20 also shares mechanical aspects with other game systems including the SPECIAL system used in the computer role-playing game Fallout, the most obvious being the "feats" gained every three character levels in most games (note that this is an extension of the d20 mechanics and, while implemented by most companies, can be dropped). The three primary designers behind the d20 System were Jonathan Tweet, Monte Cook and Skip Williams; many others contributed, most notably Richard Baker and Wizards of the Coast then-president Peter Adkison. Many give Tweet the bulk of the credit for the basic resolution mechanic, citing similarities to the system behind his game Ars Magica. Tweet, however, stated "The other designers already had a core mechanic similar to the current one when I joined the design team".[2]

To resolve an action in the d20 System, a player rolls a 20-sided die (or three 6-sided dice with the alternative bell-graphed action resolve system[3]) and adds modifiers based on the natural aptitude of the character (defined by six abilities, Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma) and how skilled the character is in various fields (such as in combat), as well as other, situational modifiers.[4] If the result is greater than or equal to a target number (called a Difficulty Class or DC) then the action succeeds. This is called the Core Mechanic and many people find it to be superior to the mechanics used by earlier editions of D&D, such as the first-edition hit tables or the second-edition AD&D "THAC0" and saving throw mechanics, which take more time to learn, and were disliked by younger players. Template:Fact This system is also consistently used for all action resolution in the d20 System: in prior games in the D&D family, the rules for different actions varied considerably in which dice were used and even whether high numbers or low numbers were preferable.

The d20 System is not presented as a universal system in any of its publications or free distributions, unlike games like GURPS. Rather, the core system has been presented in a variety of formats that have been adapted by various publishers (both Wizards of the Coast and third-party) to specific settings and genres, much like the Basic Role-Playing system common to early games by veteran RPG publisher Chaosium.

Because Dungeons and Dragons is the most popular role-playing game in the world, many third party publishers produce products designed to be compatible with that game and its cousin, d20 Modern. Wizards of the Coast provides a separate license allowing publishers to use some of its trademarked terms and a distinctive logo to help consumers identify these products. This is known as the d20 System Trademark License. The d20 System Trademark License (D20STL) requires publishers to exclude character creation and advancement rules, apply certain notices and adhere to an acceptable content policy. Games that only use the OGL are not bound by these restrictions, and several have included character creation and advancement rules, allowing them to be used as standalone products. D20STL products require a core book from Wizards of the Coast and must clearly state this. As the D20STL has changed, some companies have chosen to use the OGL by itself. All D20STL products must also use the OGL to make use of d20 System open content, but publishers may use the OGL without using the D20STL.

For a long time d20 System products using one or both licenses took a significant market share of the role-playing games industry. They have especially promoted the rise of electronic publishing, since small companies can tap the huge market potential of Dungeons and Dragons at no cost to themselves. d20 System product sales, as with the rest of the industry, are currently in flux.

Unlike the OGL, the d20 System Trademark License (D20STL) is revocable and is controlled by WotC. For critics this raises questions over the control that Wizards can exert over the open gaming movement, which is widely considered to be synonymous with the d20 SystemTemplate:Fact. WotC has the ability to alter the d20 System Trademark License at will and gives a short, 30 day "cure period" to rectify any issues with the license before termination. These changes apply retroactively to all material published under the d20 System Trademark License.

When gaming company The Valar Project, under former WotC brand manager Anthony Valtera, attempted to publish the d20 Book of Erotic Fantasy (BoEF), which contained Human sexuality, WotC altered the d20 System Trademark License in advance of publication of BoEF by adding a "quality standards" provision that required publishers comply with "community standards of decency." This subsequently prevented the book's publication under the D20STL.[6] WotC said this was done to protect its d20 System trademark, but critics claimed that it was censorship. The Book of Erotic Fantasy was subsequently published without the d20 System trademark under the OGL. Other books subsequently published under similar circumstances include Skirmisher Publishing LLC's Nuisances which also includes on its cover the disclaimer "Warning: Intended For Mature Readers Only."

The same round of changes to the license also limited the size at which the text "Requires the use of the Dungeons & Dragons Player's Handbook, Third Edition, published by Wizards of the Coast" (which is required to appear on the front or back cover of most fantasy d20 System products) could be printed, and prohibited making part of it larger than the rest. This was perceived as being aimed at the same Valar book; early mockups of the cover had the words "Dungeons & Dragons" in the above text printed much larger and in a different font from the rest, right at the top of the front cover. This could have made the book appear to be an official Dungeons & Dragons publication to a casual or uninformed observer. The published version does not have the offending text on the cover.

Criticism is also levied at the conditions for termination of the d20 System Trademark License through a breach of its terms. The license requires that, upon breach of the terms of the D20STL which includes any subsequent modifications of the license after publication of a work using the d20 System trademark, all inventory and marketing material must be destroyed. Adhering to the breach conditions is an onerous task for smaller game companies. The mere threat of this condition being imposed was a huge blow to the now defunct d20 System publisher Fast Forward Entertainment, which had released several books that used non-open WotC content due to company president James Ward's misunderstanding of the license.

Other criticism is based around the part of the d20 System Trademark License which defines "Open Game Content" to include game mechanics and purports to license it. It is widely believed that game mechanics are uncopyrightable in the USA, and according to a circular on the US Copyright Office's website,[7] "Once a game has been made public, nothing in the copyright law prevents others from developing another game based on similar principles."

One result of this has been the abandonment of the d20 System License by some publishers in favor of a simple "OGL" designation. Mongoose Publishing's licensed games based on the Conan the Barbarian property and the Robert A. Heinlein novel Starship Troopers, for example, use systems that function nearly identically to d20 but do not carry the d20 logo.

At least one company, Technomancer Press, has begun publishing d20 System-compatible material, but not under the d20 System License or OGL. Their use of the Dungeons and Dragons trademark and d20 System material is explicitly forbidden by the Open Gaming License, and this is deliberate (as is stated on their web site).[8]

Shortly after the publication of the D20STL and OGL, d20 System publications began to proliferate. Many new companies were started exclusively to publish d20 System content. This was a profitable niche for some established companies, revitalized others, and inspired the creation of new d20 System-only RPG production houses.

Sales of role-playing games had already been in decline, and the popularity of the d20 System motivated companies to refocus on those products at the expense of their own games. Some companies (notably Alderac Entertainment Group, with 7th Sea and Legend of the Five Rings) experimented with d20 System versions of existing creative properties.

The d20 System jump-started the fledgling PDF role-playing publishing industry. Since many of the d20 System publishers were small, amateur companies started by fans, publishing as PDF offered a cheap and easy way of getting published, without the minimum returns required by professional ventures. Some of these companies became profitable and even broke through into offset print (as opposed to print on demand) runs.

While various manifestations of the d20 System still compose the single largest marketshare in role-playing games, various sources (including Kenneth Hite and Game Trade Magazine) report an overall decline in RPG sales. Some attribute this decline, at least partially, to declining d20 System game sales. An overcrowded market is also put forward as a reason, since the proliferation of d20 System supplements divides the budgets of retailers, distributors and hobbyists.